Conductometric Study of the Thermodynamics of Micellization of Sodium dodecylsulfate (SDS) in the Presence of Some Aromatic Ammonium Salts

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1 Regular Article PHYSICAL CHEMISTRY RESEARCH Published by the Iranian Chemical Sciety Phys. Chem. Res., Vl. 5, N. 3, , September 217 DOI: /pcr Cnductmetric Study f the Thermdynamics f Micellizatin f Sdium ddecylsulfate (SDS) in the Presence f Sme Armatic Ammnium Salts O. Owymi*, J. Funs Olrunymi and O. Emmanuel Olaye Chemistry Department, Obafemi Awlw University, Ile Ife, Nigeria (Received 29 Nvember 216, Accepted 16 April 217) The effect f three rganic armatic salts - Phenyltrimethylammnium (PhTMAB), benzyltrimethylammnium (BzTMAB) and benzyltrimethylammnium (BzTEAB) brmides - n the micellizatin f sdium ddecylsulfate has been investigated by cnductmetric methd. The critical micelle cncentratin () values were fund t decrease with increase in the cncentratin f the armatic ammnium salts. Thermdynamic parameters f micellizatin were btained frm measurement in the presence f.1 M f the three quaternary armatic ammnium salts in the temperature range K at 5 K intervals. The Gibbs free energy values were generally negative in the presence f the armatic salts and slightly varied with temperature. The micellizatin prcess was bth enthalpy and entrpy driven. Enthalpy-entrpy cmpensatin was bserved fr all the system with a cnstant cmpensatin temperature values arund 32 K and negative intrinsic enthalpy f cmpensatin. Keywrds: Micellizatin, Armatic ammnium salts, Thermdynamic parameters, Enthalpy-entrpy cmpensatin INTRODUCTION Aggregate and micelle frmatin by surfactant invlves a cmplex interplay f different intermlecular frces with cntributins frm the slvents, mlecular structure f the mnmer, temperature, pressure and the presence f ther c-slutes [1]. Mst f the applicatins f surfactants in such area as detergency, pharmaceutical and csmetic frmulatins, enhanced and tertiary il recvery, agrchemicals, slubilisatin, bichemical research, etc. require the additin f ther substances such as anther surfactant, plar rganic cmpunds, and rganic salts (als ften called hydrtrpes) with a view t imprving the perfrmance f the surfactants [2-5]. The presence f rganic salts in surfactant slutins can significantly affect the micellizatin prcess by decreasing the electrstatic repulsin amng the head-grups f surfactant mlecules and thrugh hydrphbic interactins. These effects lead t a cncmitant decrease in the f *Crrespnding authr. wymi@auife.edu.ng the surfactant [6-9]. The micellizatin f sdium ddecylsulfate (SDS) in the presence f tetraalkylammnium salts (R 4 N + Br - ; R = H, CH 3, C 2 H 5, C 3 H 7, and C 4 H 9 ) has been extensively studied [1-12]. The salts were shwn t cause a remarkable decrease in the and enhanced slubilisatin f the surfactants. In additin, (C 4 H 9 ) 4 N + Br - was fund t prduce clud pint phenmenn in the micellizatin f lithium ddecylsulfate [13]. Althugh, the hydrtrpic prperties f phenyltriakylammnium salts are well dcumented [14-16], the effect f these salts n the micellar prperties f surfactants is scantily reprted [16]. In this wrk, the effects f phenyltrimethylammnium brmide (PhTMAB), benzyltrimethylammnium brmide (BzTMAB) and benzyltriethylammnium brmide (BzTEAB) n the micellar prperties f SDS have been studied using cnductmetric methd. The effect f the structural differences in the rganic salts n the micellar prperties f SDS was als investigated. Thermdynamic prperties f the micellizatin prcess in the presence f.1 ml dm -3

2 Owymi et al./phys. Chem. Res., Vl. 5, N. 3, , September 217. Br - N + N + Br - N + Br - PhTMAB BzTMAB BzTEAB Scheme 1. Structures f the armatic ammnium salts used in this wrk f each f the salts were btained in the K temperature range at 5 K intervals. EXPERIMENTAL Materials PhTMAB, BzTMAB and BzTEAB frm Sigma-Aldrich, USA with stated purity > 99% were used as received, SDS was f mlecular bilgy grade als frm Sigma-Aldrich, USA. The structures f the rganic salts are shwn in Scheme 1. Methd Cnductmetric titratin methd invlving the titratin f a knwn vlume f a cncentrated surfactant slutin prepared in different armatic rganic salt slutins, int a fixed vlume f aqueus slutins f the respective rganic salts cntained in a thermstatted beaker was emplyed. Surfactant slutins were prepared by disslving accurately weighed quantities f surfactants in requisite vlumes f distilled water. Distilled water (cnductivity < 3 μs cm -1, at 298 K) was used fr all purpses. All measurements were made at 298 K (except fr the temperature dependent studies) in a thermstatted water bath Grant Y14 frm Grant Instruments (Cambridge) Ltd, England, maintaining the temperature cnstant within ±.1 K. The cnductivity measurements were made as a functin f ttal SDS cncentratin at.5,.1,.2,.3,.4 and.5 ml dm -3 f the respective armatic rganic salt. All the cnductivity plts shwed a linear variatin in the specific cnductivity values as ttal surfactant cncentratin increases in bth the pre-micellar and pstmicellar regins. Measurements were repeated three times fr reprducibility. The uncertainties in determinatin errr did nt exceed 3%. RESULTS AND DISCUSSION Critical Micelle Cncentratin The values f the f SDS in the presence f different cncentratins f the armatic ammnium salts in aqueus systems were determined by cnductmetric methd. The plt f the specific cnductivity against SDS cncentratin in pure aqueus system at 298 K is shwn in Fig. 1. Typically, the cnductmetric plts can be linearly crrelated t the surfactant cncentratins in bth the preaggregate and the pst-aggregate regins with the pint f intersectin between the tw straight lines given the. The degree f cunter-in dissciatin, δ, f the micelle in the presence f the rganic salts was btained frm the rati f the slpes in the pst-micellar regin t that f the premicellar regin. The f the surfactants was fund t decrease in the presence f these salts with the decrease being dependent n the cncentratin f the rganic salts as shwn in Fig. 2 fr all the three hydrtrpic salts. The bserved decrease in the f SDS in the presence f these rganic ammnium salts can be explained in terms f the electrstatic interactin between the psitively charged ammnium in and the negatively charged (-OSO 3 - ) head grup f the surfactant. Such interactin will reduce the repulsin between the plar head grup f SDS micelles, which in turn will increase the stability f the inic micelle. In additin, the rganic mieties f the rganic ammnium salts interact with the alkyl chain in SDS thrugh hydrphbic bnding. These tw factrs cmbined t cause a marked reductin in the f the surfactant in the presence f these salts. The variatin f the f SDS with the cncentratins f the rganic ammnium salts is displayed in Fig. 2 belw and the micellizatin parameters btained frm the cnductmetric studies are included in Table

3 Thermdynamic f Micellizatin f SDS in Aqueus Slutins f Sme Armatic Ammnium Salts/Phys. Chem. Res., Vl. 5, N. 3, , September Specific cnductivity (S cm -1 ) [SDS] (ml dm -3 ) Fig. 1. Variatin f specific cnductivity with [SDS] in pure aqueus system at 298 K (mm) Cncentratin f hydrtrpic salts (M) Fig. 2. f SDS as a functin f hydrtrpe cncentratins. ( ) PhTMAB, ( ) BzTMAB, ( ) BzTEAB. 533

4 Owymi et al./phys. Chem. Res., Vl. 5, N. 3, , September 217. Table 1. Critical Micelle Cncentratin () and Degree f Cunter-in Dissciatin δ, Obtained frm Cnductmetric Study f the Micellizatin f SDS in Varius Cncentratins f the Armatic Ammnium Salts at 298 ±.1 K a [Salt] (M) PhTMAB BzTMAB BzTEAB ( mm) δ a The standard uncertainties are u () = ±.9%; u (δ) = ±.75%. The decrease in in the presence f the armatic ammnium salts is mre prnunced with BzTMAB and BzTEAB than in the presence f PhTMAB. This can be attributed t the presence f methylene (CH 2 ) grup in the frmer which increases the hydrphbicity f the tw salts cmpared t PhTMAB. This increased in hydrphbicity leads t the marked reductin in f SDS bserved. The change in head grup frm trimethylammnium in 534

5 Thermdynamic f Micellizatin f SDS in Aqueus Slutins f Sme Armatic Ammnium Salts/Phys. Chem. Res., Vl. 5, N. 3, , September (ml dm -3 ) Temperature (K) Fig. 3. Temperature dependence f f SDS in the presence f.1 M armatic ammnium salts. ( ) PhTMAB, ( ) BzTMAB, ( ) BzTEAB ln Temperature (K) Fig. 4. Variatin f lnχ with temperature accrding t the fitting functin in Eq. (4). ( ) PhTMAB, ( ) BzTMAB, (x) BzTEAB. 535

6 Owymi et al./phys. Chem. Res., Vl. 5, N. 3, , September 217. BzTMAB t triethylammnium in BzTEAB did nt result in any significant difference in the effect n by these tw salts; this is an indicatin that the rganic catins mst prbably apprached the micellar surface frm the phenyl end rather than frm the alkylammnium end. THERMODYNAMICS OF MICELLIZATION The values f the surfactant in the presence f the rganic additives shwed a minimum in the plts f the variatin f with temperature (Fig. 3). The minimum in these curves is nticed between 298 K and 33 K, which appears t cnfrm t the general finding that the minimum fr inic surfactants with 12 carbn chain lengths is clse t rm temperature [17,18]. The relevant thermdynamic parameters, such as G M, and, f micellizatin H M S M were determined n the basis f the phase separatin mdel [19]. Accrdingly G 2 RT ln (1) M where χ is the value f expressed n a mle fractin basis, defined as; [Salt] The cnstant term (55.55) in Eq. (2) is the mlar cncentratin f water in aqueus system. The values f the degree f cunter-in dissciatin (δ) did nt change appreciably with temperature as can be seen in Table 3; hence, the average values f δ were used in cmputing the thermdynamic parameters. The enthalpy f micellizatin can be btained frm the temperature dependence f the using the Gibbs-Helmhltz Equatin. G T H M T 2 RT T ln 2 M 2 T The temperature dependence f the lnχ was fitted t the (2) (3) equatin derived by Kim and Lim fr the temperature dependence f [2]; ln A BlnT C T (4) where the A, B and C have been determined by a leastsquare regressin analysis. The fitting f the functin in Eq. (4) t the variatin f lnχ with temperature fr the micellizatin f SDS in the presence f.1 ml dm -3 f each f the rganic salts is shwn in Fig. 4. The values f A, B, and C btained frm the regressin analysis f equatin 4 are listed in Table 2. The was then determined by 1 S M H M GM T S M Varius thermdynamic parameters f micellizatin f SDS in the presence f.1 ml dm -3 f the btained frm the equatins abve are listed in Table 3. The calculated values fr the micellizatin f SDS G M in the presence f the three salts decreases with increase in temperature, albeit at a slw rate. This bservatin had been attributed t enthalpy-entrpy cmpensatin effect. The free energy is negative in the whle temperature range studied fr micellizatin f SDS in the presence f the three rganic ammnium salts. The average values being , and kj ml -1 fr PhTMAB, BzTMAB and BzTEAB respectively with the slight differences were attributed t the structures f the rganic ammnium salts. It can be seen frm Table 3 that the micellizatin prcess is bth entrpic and enthalpic driven with changes frm entrpic t enthalpic as temperature increases. Similar bservatins have been reprted fr different surfactants [13,21-23]. There is a very gd crrelatin between the change in enthalpy and change in entrpy f micellizatin with crrelatin cefficients clse t unity fr the micellizatin f SDS in the presence f the three rganic ammnium salts studied with regressin equatin: H H T S (6) M C M The equatin abve suggests that the enthalpic change cnsists f tw cmpnents. The first cmpnent H is (5) 536

7 Thermdynamic f Micellizatin f SDS in Aqueus Slutins f Sme Armatic Ammnium Salts/Phys. Chem. Res., Vl. 5, N. 3, , September 217. Table 2. Regressin Parameters fr the Dependence f lnχ n Temperature Accrding t Eq. (4) a Salt A B C R 2 PhTMAB ± ± ± BzTMAB ± ± ± BzTEAB ± ± ± a The uncertainties in the parameters are based n 95% cnfidence limit. Table 3. Thermdynamic Parameters fr the Micellizatin f SDS in the Presence f.1 M Armatic Ammnium salts at Different Temperatures a T (K) (mm) δ G M (kj ml -1 ) H M (kj ml -1 ) S M (kj ml -1 ) PhTMAB BzTMAB BzTEAB a The standard uncertainties are u () = ±.9%; u (δ) = ±.75%; u ( ) = ±.5%; u ( =.5%); u ( S M =.71%). G M H M

8 Owymi et al./phys. Chem. Res., Vl. 5, N. 3, , September H (kj ml -1 ) S (J ml -1 K -1 ) Fig. 5. Typical enthalpy-entrpy plt fr the micellizatin f SDS in the presence f.1 M PhTMAB. independent f the entrpy change and the secnd ne T S is prprtinal t it. The slpe T c is knwn as the C M cmpensatin temperature and interpreted as a measure f the deslvatin prcess f micellizatin. The values btained was essentially the same (33.54 ± 2.46, ± 2.52 and 32.9 ± 1.74 K) fr the micellizatin prcess in the presence f the three rganic salts studied which implies that the slight structural differences in the rganic additives has n effect n the deslvatin part f the micellizatin prcess. The values f the intrinsic enthalpy gain, H, were ±.19, ±.14 and ±.15 kj ml -1, respectively, in the presence f PhTMAB, BzTMAB and BzTEAB. The negative values f this intrinsic enthalpy gain indicate that the micellizatin prcess is favured with r withut any entrpic cntributin. A typical enthalpy-entrpy cmpensatin plt is shwn in Fig. 5. CONCLUSIONS This study shws that the micellizatin f SDS is significantly affected by the armatic ammnium salts by causing a marked reductin in the f the surfactants. The decrease in was fund t be dependent n the cncentratin f the salts. The presence f methylene grup in tw f the salts als cntributed t the bserved reductin in. Hwever, head grup mdificatin f the salts appeared nt t impact significantly n the reductin in f the surfactant. REFERENCES [1] Tadrs, T. F., Applied surfactants: principles and applicatins. VCH Weinheim, 25, p [2] Hffmann, H.; Hrbaschek, K.; Witte, F., Vesicle phases with semiplar additives. J. Cllid Inter. Sci. 21, 235, 33-45, DOI: 1.16/jcis

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